1. Field of the Invention
The present invention relates to a coating liquid for an electrophotographic photoreceptor, an electrophotographic photoreceptor which is prepared using the coating liquid, and electrophotographic image forming method and apparatus using the electrophotographic photoreceptor. In addition, the present invention also relates to an electrophotographic process cartridge using the electrophotographic photoreceptor.
2. Discussion of the Background
Conventionally, inorganic photosensitive materials such as Se, CdS and ZnO have been used for electrophotographic photoreceptors. In addition, organic photosensitive materials have been developed and used for electrophotographic photoreceptors because of having advantages of high photo-sensitivity and heat stability and less toxicity over the inorganic photosensitive materials. Therefore, at the present time photoreceptors using an organic photosensitive material are typically used for current copiers and printers.
In electrophotographic image forming apparatus such as printers, copiers and facsimiles, a series of image forming processes such as charging, light irradiating, developing and transferring processes is typically performed. Such image forming apparatus at least includes a charger, an imagewise light irradiator, an image developer (particularly, a reverse image developer), a transfer device and a photoreceptor. The image forming apparatus having such a constitution tend to produce abnormal images after long repeated use.
Recently, digitization rapidly progresses and is applied to image forming apparatus such as printers and copiers. Therefore, photoreceptors which are sensitive to laser light (i.e., monochromatic light) emitted by a laser diode, are needed to be used for such digital image forming apparatus. Not to produce abnormal images such as moiré is one of the requisites of the photoreceptors, and an undercoat layer formed between an electroconductive substrate and a photosensitive layer plays a important role in preventing the moiré problem. For example, a technique in which a particulate metal oxide or a particulate organic material is included in an undercoat layer is proposed to prevent the moiré problem.
The following techniques have been proposed with respect to the undercoat layer.    (1) Japanese Laid-Open Patent Publication No. (hereinafter referred to as JOP) 11-15181 discloses a photoreceptor which includes an aluminum or aluminum alloy substrate having a surface which is subjected to an anodizing treatment, followed by a mechanical polishing treatment and a sealing treatment using hot water or a sealing treatment while humidifying, and a photosensitive layer located on the substrate;    (2) JOP 10-301314 discloses a photoreceptor which has an electroconductive substrate, and an undercoat layer and a photosensitive layer, which are overlaid on the substrate, wherein a composition in which a kind of organoalkoxy siloxane is mixed with a colloidal alumina and which is crosslinked upon application of heat thereto is used as the undercoat layer;    (3) JOP 10-90931 discloses a photoreceptor which has an electroconductive substrate, and an undercoat layer and a photosensitive layer, which are overlaid on the substrate, wherein the undercoat layer includes a resin and titanium oxide subjected to a heat treatment;    (4) JOP 5-204181 discloses a photoreceptor which has a substrate, and an electroconductive polyaniline layer and a photosensitive layer, which are overlaid on the substrate; and    (5) JOP 8-44096 discloses a photoreceptor which has a substrate, and an undercoat layer and a photosensitive layer, which are overlaid on the substrate, wherein the undercoat layer includes a heat-crosslinked resin at a volume content ratio of from 0.5 to 0.6 and titanium oxide having an average particle diamter not greater than 0.4 μm;    (6) JOP 9-34152 discloses a photoreceptor which has an electroconductive substrate, and an undercoat layer and a photosensitive layer, which are overlaid on the substrate, wherein the substrate is made of aluminum, an Al—Mn alloy, an Al—Mg alloy or an Al—Mg—Si alloy, and the undercoat layer includes a compound selected from the group consisting of metal alkoxides, organic metal chelate compounds, silane coupling agents and reaction products thereof;    (7) JOP 9-292730 discloses a photoreceptor for reverse development in which a photosensitive layer is formed on an anodized surface of an electroconductive substrate made of aluminum or an aluminum alloy, wherein the anodized surface has a profile in which an interval Sm between a projected portion and an adjacent projected portion is from 0.3 to 250 μm; the maximum height Rt of the projected portions is from 0.5 to 2.5 μm; and the gloss of the anodized surface is not less than 60 gloss; and    (8) JOP 10-83093 discloses a photoreceptor in which an undercoat layer is formed between an electroconductive substrate and a photosensitive layer, wherein the undercoat layer includes a particulate titanium oxide having a surface including at least zirconium oxide.
In the above-mentioned photoreceptors (1) to (8), various constitutions and/or materials are applied to the undercoat layer or an anodic oxide layer is applied as the undercoat layer to prevent injection of holes into a photosensitive layer (or a charge generation layer), resulting in prevention of occurrence of black spot images. However, the above-mentioned photoreceptors are not satisfactory because recently photoreceptors do not meet the current requirements therefor, such as producing high quality images and having high durability.
As mentioned above, photoreceptors are needed not to produce moiré which is caused by light interference. In order to avoid the moiré problem, photoreceptors including a metal oxide such as titanium oxide in their undercoat layer are typically used.
An undercoat layer including titanium oxide is typically formed by coating a coating liquid, in which titanium oxide is dispersed in a resin and a dispersion medium, on an electroconductive substrate. Since metal oxides such as titanium oxide have a specific gravity much greater than a resin and a dispersion medium used in the coating liquid, the dispersion stability and coating properties of the resultant coating liquids are typically poor. Therefore a uniform undercoat layer cannot be formed because metal oxides included in the coating liquids tend to easily aggregate. Therefore, aggregated particles tend to be included in the resultant undercoat layers. Alternatively there is a problem in that the productivity of the coating liquids is low because such coating liquids as including an aggregated metal oxide are typically disposed of.
Because of these reasons, a need exists for an electrophotographic photoreceptor which can produce high quality images for a long period of time without producing abnormal images such as black spots.